Rankine cycle power plants operating with an organic working fluid are known in the art. Such a power plant comprises a boiler for vaporizing the working fluid, a turbine responsive to vaporized working fluid produced by the boiler for expanding the vapor and producing work, a generator coupled to the turbine for converting the work produced thereby into electrical energy, and a condenser for condensing expanded vaporized working fluid exhausted from the turbine and producing condensate that is returned to boiler either by pump or under the influence of gravity. A power plant of this type, hereinafter refered to as a power plant of the type described, is commercially available by Ormat Turbines Ltd. and is described in the Patent literature, in, for example, U.S. Pat. No. 3,040,528.
Power plants of the type described are in current use throughout the world for supplying electrical energy for telecommunication relay stations, for example, and other installations where the power required is in the 300-3000 W range, and reliability is critical. Reliability is enhanced in a power plant of the type described by utilizing an air cooled condenser, by mounting the turbine and generator on a common shaft (turbogenerator) and hermetically enclosing these components in a cannister, by diverting a small portion of the condensate from the condenser to the bearings of the turbogenerator in order to effect long term operations without wear, and by controlling the voltage of the generator by on/off operation of the fuel supplied to the boiler.
Conventionally, the working fluid is a fluorinated hydrocarbon such a Freon, trichlorobenzene, etc; and the turbine operating conditions for trichlorobenzene are about 160 deg. C. and below atmospheric pressure while the air cooled condenser operates at 70 deg. C. and much less than atmospheric pressure. Under these conditions of temperature and pressure conventional working fluids are stable with time in the presence of copper, stainless steel, low carbon steel, aluminum and brass, or metals that are used in the construction of the power plant of the type described. In addition, these conventional working fluids have thermodynamic proprieties which permit the working fluids to be used advantageously in a Rankine cycle power plant of the type described.
In order to up-size power plants of the type described by an order of magnitude or more so as to utilize low grade heat sources, such as waste heat, geothermal heat, and solar heat, a larger turbine operating at higher pressures and temperatures must be utilized if the size of the turbine is to be kept to reasonable dimension. Conventional working fluids such as fluorinated hydrocarbons prove to be unstable with time in the presence of the usual metals found in power plants when the operating temperature is in the range 300-400 deg. C. Moreover, as the capacity of power plant of the type described increases, the bearing loads on the turbine also increase; and it is not always practical to construct both the turbine and generator as a single unit enclosed in a hermetically seal cannister as is done conventionally on small capacity power plant.
As a consequence, a 750 kW or larger power plant of the type described (such as would be operable to generate power from a waste heat or a geothermal source, for example) would normally have a single stage turbine mounted in its own housing with the rotor journaled in bearings mounted in housing, and a seperately housed generator coupled to the output shaft of the turbine. Thus, efficient seals are required, particularly if the turbine housing is under a vacuum, which would permit ambient water vapor to leak into the house. With many types of conventional working fluids, water vapor chemically reacts with the working fluid in the temperature range utilized and produces constituants that are corrosive to the various metals used in constructing the power plant. Increased maintenance and expense in operation result.
Another disadvantage with many conventional working fluids is their relatively high freezing point. For example, the freezing point of commercial trichlorobenzene is about 10 deg. C. which would render the power plant incapable of "cold-starting" in many places in the world. While expedients ar known by which mixtures of different working fluids can be used in order to suppress the freezing point of the combination, this approach to solving the problems encountered in using power plants of the type described in cold regions is not always satisfactory.
It is therfore an object of the present invention to provide a new and improved organic working fluid suitable to a power plant of the type described wherein the working fluid is more stable and provides improved results.